1. Field of the Invention
The present invention is directed generally to processes for forming layers of fibrous material and, more specifically, to a wetlay process for manufacturing highly-oriented fibrous mats.
2. Description of the Related Art
Wetlay processes for manufacturing fibrous mats have typically been directed to the use of long glass, mineral wool or carbon fibers on both inclined wire wetlay machines and on rotary formers (cylinder machines). Typical wetlay processes involve injecting stock containing a plurality of fibers into the headbox of a wetlay machine. Suction under a wirebelt draws fibers within the stock toward the wirebelt to ultimately form a fibrous mat. In general, fiber orientation is often controlled to make it as random (square or 1:1 strength profile) as possible. Various existing patents depict machinery improvements to prevent shear boundary layers which might tend to form small areas of oriented fiber. For example, such shear boundary layers often form at the side walls of the headbox or between adjacent stock flows into the headbox. This is because inadvertent fiber alignment in the machine direction reduces transverse (cross machine) mat strength.
Typical glass mat machines may produce a maximum of 1.4 to 1 machine direction (MD) to cross-machine direction (CD) orientation (58% MD orientation), because the suction (forming) wire speed is higher than the incoming water speed. A few machines have been known to orient at a 4 to 1 ratio (80%), while even fewer machines have been known to orient at a 6 to 1 ratio (6/7=85.7%).
In general, degree of orientation is measured as:
[MD/CD]/[(MD/CD)+1]
where the span between the jaws of the tensile tester is longer than the longest reinforcement fiber in the structure to avoid bridging the gap.
All prior attempts, however, have failed to produce a greater than 90% wetlay orientation (9 to 1 MD to CD strength ratio or greater). As such, there exists a need to develop fibrous mats having the strength characteristics associated with a mat having greater than 90% wetlay orientation. In addition, many prior attempts to improve existing machinery required the use of nozzles to increase fiber velocity. Such prior attempts have not, however, readily lent themselves to retrofitting existing machinery. As such, there is currently a need to develop a cost-effective and efficient system to retrofit existing machinery so that they are capable of providing mats with at least a 90% wetlay orientation.
In accordance with the present invention, the invention includes a method of producing highly-oriented fibrous mats having at least a 90% machine direction orientation including the steps of producing a thickened solution containing a plurality of suspended fibers, introducing the thickened suspension into a headbox of a wetlay machine and decelerating the fiber suspension to a velocity less than wirebelt operating velocity, and applying suction through the wirebelt to orient and pin the fibers on the wirebelt.
The present invention also includes a method of retrofitting an existing headbox of a wetlay machine so as to produce highly-oriented fibrous mats, including the steps of increasing head level within the headbox to increase headbox stock capacity, and accelerating operating velocity of a wirebelt within the wetlay machine beyond an operating velocity of stock entering the headbox.
The present invention also includes end products made of a plurality of mats, each of the mats including a plurality of discontinuous reinforcement fibers having at least a 90% machine direction orientation.